1. Field of the Invention
This invention relates to a fish rearing system and, more particularly to such a system including a primary filtration means and a biological filter in which effluent water from a fish rearing tank can be treated and reused in the rearing tank.
2. Description of the Prior Art
There is an increasing demand for sport and commercial fishes. This demand can only be satisfied by means of hatcheries of similar operations. These fish rearing systems require large amounts of water to operate on an appropriately large scale. Waters have normally been drawn from rivers or lakes. Such waters may vary rapidly in quality and thus endanger fish stocks.
Reuse or recycling of water is becoming increasingly desirable because there is a diminishing water supply of suitable quality; the risks to fish stocks occur due to the use of low quality water, variations in flow rate, solid content and disease levels. In addition pollution abatement costs are high for single pass systems.
In theory at least, reuse and recycling of water can provide; a substantial reduction in water requirements; optimization of fish growth rate by regulation of the water temperatures; control of disease by sterilization of intake water; and reduction of overall water supply and pollution abatement costs.
The essential requirements of a water reconditioning system for a fish rearing system must include means to control the level of suspended solids such as fish feces and unused fish food, and ammonia and carbon dioxide. Furthermore water must be returned to the fish rearing tank of the system with sufficient dissolved oxygen for fish respiration.
Burrows et al in the Progressive Fish Culturist, 30: 123 to 136, 1968 described the use of a downflow flooded filter consisting of a four foot layer of half inch to three inch rock covered with a one foot layer of one quarter to three quarter inch oyster shells. Unsettled hatchery effluent was applied at 0.83 gallons per minute per square foot of filter area. This unit has found practical application at the Dworshak Fish Hatchery in Oregon and has been installed by the Canadian Fisheries Service at the Capilano Hatchery in North Vancouver. However oyster shells were omitted from the latter application. Extensive operational problems have been encountered at Dworshak. These have been due principally to the blinding of the filters, to algae growth and to poor flow maintenance.
In technical Report No. 67, New Mexico State University, 1970, 119 pp. Gigger et al described the use of filters consisting of three quarter inch gravel or plastic media available under the trade mark SURFPAC in a semi-pilot scale test with six inch rainbow trout as the test species. Flow rate ranged from 3.5 to 5.34 gallons per minute per square foot and bed volumes were replaced at 1,39 to 4.19 minute intervals. The filter removed 150 to 200 mg. of ammonia nitrogen per hour per cubic foot of media. However the exposure time of fish in these tests was too short to assess long time ammonia toxicity effects. Furthermore no nitrite measurements were made on the test unit.
McCrimmon et al in the Progressive Fish Culturist 28: 165 to 170, 1966 described a laboratory scale water recirculation unit for holding trout. The unit involves sand filtration and carbon contacting of the recirculated water. Scott et al in Journal of the Fisheries Research Board of Canada 29: 1071 to 1074, 1974 shows that hatchery rearing water could be recycled by filtering through a two stage high rate filtration system using large particle anthracite coal. In their experiments the fish grew rapidly with negligible mortality. pH was maintained in the range of 6.5 to 7.5 but ammonia levels fluctuated widely in the range 4 to 22 milligrams of ammonia nitrogen per liter. Problems were also encountered with backwashing the filter.
Mayo et al, in "A Study for Development of Fish Hatchery Water Treatment Systems" prepared for Walla Walla District Corps of Engineers in co-operation with the U.S. Department of the Interior, U.S. Bureau of Sports, Fisheries and Wildlife, by Kramer, Chin and Mayo, Seatlle, 1972, 42 pp., compared biological filter systems using one inch and three and one half inch Koch rings and one quarter inch foamed polystyrene (stryofoam) pellets as his filter medium. The systems were compared with biological systems designed on the activated sludge tank technique. The tests showed that all systems -- biological filter or activated slude --could be used for pollution control abatement but filter systems were more reliable when water reuse was a primary objective. Problems with nitrite toxicity occurred but these could be avoided by using controlled programs of management which would allow gradual increase in the fish load, thereby limiting the level of nitrite produced in the system.